Head-Mounted Displays (HMDs) are being developed for a range of diverse uses, including military, commercial, industrial, fire-fighting, and entertainment applications. For many of these applications, there is particular value in forming a virtual image that can be visually superimposed over the real-world image that lies in the field of view of the HMD user. Optical imaging light guides convey image-bearing light to a viewer in a narrow space for directing the virtual image to the viewer's pupil and enabling this superposition function.
In such conventional imaging light guides, collimated, relatively angularly encoded light beams from an image source are coupled into a planar waveguide by an input coupling such as an in-coupling diffractive optic, which can be mounted or formed on a surface of the planar waveguide or buried within the waveguide. Such diffractive optics can be formed as diffraction gratings, holographic optical elements or in other known ways. For example, the diffraction grating can be formed by surface relief. After propagating along the waveguide, the diffracted light can be directed back out of the waveguide by a similar output grating, which can be arranged to provide pupil expansion along one dimension of the virtual image. In addition, a turning grating can be positioned along the waveguide between the input and output gratings to provide pupil expansion in an orthogonal dimension of the virtual image. The image-bearing light output from the waveguide provides an expanded eyebox for the viewer.
Although conventional imaging light guide arrangements have provided significant reductions in bulk, weight, and overall cost of near-eye display optics, overall efficiency of the gratings is often limited by optical losses occurring at each grating interface. Since each grating area can only be fully optimized for one specific field angle and for one specific wavelength, performance across the field of view of the virtual image or across the visual spectrum of the same virtual image can vary greatly. This is true also of the turning grating that directs light that is traveling from the in-coupling to the out-coupling diffractive optics. Because an appreciable amount of input light energy is lost as the light encounters each diffractive optic, the input image source must be bright enough to compensate for lost brightness in the virtual image presented to the viewer.
Thus, it can be appreciated that there is a need for improved designs of image bearing light guides that still provide the desired pupil expansion, but provide enhanced efficiently in head-mounted displays.
In considering a light guide design used for imaging it should be noted that image-bearing light traveling within a waveguide is effectively encoded by the input coupling, whether the coupling mechanism uses gratings, holograms, prisms, mirrors, or some other mechanism. Any reflection, refraction, and/or diffraction of light that takes place at the input must be correspondingly decoded by the output in order to re-form the virtual image that is presented to the viewer.
A turning grating, placed in an intermediate position between the in-coupling and out-coupling diffractive optics, is typically chosen so that it does not induce any change on the encoded light. Preferably, the turning gratings redirect ray bundles within the waveguide, but do not change the encoded angular information of the virtual image. The resulting virtual image in such a designed system is not rotated. Further, if such a system did introduce rotation to the virtual image, it would do so non-uniformly across different field angles and wavelengths of light, thus causing unwanted distortions or aberrations in the resulting virtual image.
U.S. Pat. No. 6,829,095 by Amitai entitled “Substrate-Guided Optical Beam Expander” discloses input and output couplings in the form of mirrors that reflect sets of image bearing light beams into and out of a planar waveguide. The output coupling is divided into an array of reflective surfaces for expanding the exit pupil along one dimension. An intermediate array of reflective surfaces, referred to herein as a turning mirror, provides for expanding the exit pupil in an orthogonal dimension. The various input, output, and intermediate reflective surfaces are matched to each other to preserve the desired angular orientations of the image bearing beams.
One-dimensional (1-D) pupil-expansion guides of the type Amitai describes, however, have proved to be costly and difficult to fabricate. Extending this concept to 2-D beam expansion, using an array of mirrors oriented at a second set of angles, greatly complicates fabrication tasks that are already formidable and introduces alignment requirements that would be extremely difficult to satisfy.
Thus, both the turning gratings and the turning mirrors have been matched and oriented to work with similar types of input and output couplings, i.e., gratings with gratings and mirrors with mirrors. However, if a turning grating were used to redirect light that has been input using a mirror or a prism, this would produce unwanted effects in the resultant virtual image. As one consideration, with any type of reflective surface used in imaging, there can be unwanted reversal/rotation of the in-coupled light.
From the perspective of manufacturability, the use of diffractive optics to input and output the image-bearing light beams into and out of the waveguide can simplify a number of optical design problems. There is still, however, a need for an optical solution that allows better performance, increased efficiency, and compact packaging arrangements for redirecting light within a planar waveguide from the input couplings to the output couplings.